1. Field of the Invention
The present invention relates to a measuring device and a measuring method for capacitive pressure and/or temperature measurement, such as, for example, for tire pressure control systems. Although it can be used in principle in any pressure and temperature measuring devices, the present invention and its underlying situation will be described below with reference to wireless pressure and temperature measuring devices with use of transponders and here particularly for tire pressure control systems.
2. Description of the Background Art
Transponders are microelectronic elements, which are designed to be able to store information. For this purpose, transponders have an antenna array to enable contactless reading out and modification of the stored information. In the case of transponders, an electromagnetic signal transmitted by the base station is received by the transponder and demodulated.
Active, semipassive, and passive transponders are differentiated depending on the design of their energy supply. In contrast to active transponders, passive transponders do not have their own energy supply, so that the passive transponder obtain the energy necessary for demodulating and decoding the received electromagnetic signal from the electromagnetic signal itself. This occurs by absorption modulation of the transmitted electromagnetic waves. In currently employed passive 110-140 kHz transponder systems, this is realized in the electromagnetic near field by inductive coupling. The energy range achieved thereby is within the range of a few centimeters to about half a meter and depends inter alia on the specific national HF regulations.
Passive transponders are employed in the field of contactless communication for identification (RFID=Radio Frequency Identification), in tire pressure control systems, and the like. In the case of a tire pressure control system, typically piezoresistive or capacitive pressure sensors are employed for determining tire-specific parameters, particularly tire pressure and tire temperature, whereby the present invention relates also to capacitively designed pressure sensors.
The absolute value of the capacitance of the pressure sensor, which corresponds to the actual tire pressure of the automobile tire, is to be determined by the capacitive pressure sensor. The tire temperature is determined in a similar way with the internal reference capacitance. In this case, the temperature-dependent charge of the reference capacitor is evaluated. A voltage is applied to the pressure sensor to measure this capacitance. The charge stored in the capacitor of the pressure sensor, which thereby contains information on the actual tire pressure, is subsequently determined in an AD converter using suitable converter methods. The challenge in determining the actual tire pressure from the charge stored by this capacitor now is the highest measuring accuracy possible, a resolution of less than one per thousand being required in current tire pressure control systems.
A/D converters used today or conventional corresponding converter methods in fact perhaps achieve such a high accuracy in the resolution, but do not take into account the integratability of the pressure sensor. In particular, the capacitive pressure sensor for tire pressure measurements as a component of an integrated circuit, which is disposed on a wheel of a vehicle or vulcanized into its rubber material, must be as small as possible and therefore made area-optimized. A/D converters and capacitive pressure sensors in use today, which are designed for a high accuracy, are not suitable here or only suitable conditionally when designed for this. Moreover, these have a relatively long converter time to determine the capacitance of the capacitive pressure sensor and thereby the actual tire pressure. A long converter time, however, is directly associated with a high energy consumption, which is essential to avoid, however, particularly in tire pressure control systems in which the transponder must obtain the energy from the electromagnetically transmitted signal.
A pressure and temperature sensor with wireless transmission of measured data for microelectronic circuits is described in the article by Werner Schulz, “A wireless integrated pressure sensor” in the journal Elektronik, 7/2000, page 54. As a result of the low power consumption, the current supply can be obtained here from the field of a base station up to a meter away.
In the German textbook by Tietze and Schenk “Semiconductor Circuit Technology,” 11th edition, pages 1059-1060, a generally known so-called dual-slope method is described, furthermore, by means of which the capacitance of a capacitor can be determined.